Band-stop filters are needed to suppress undesired frequency bands. Such filters can be used, e.g., advantageously when one of two frequency bands lying close to each other in one wireless transmission standard is to be blanked, wherein, in addition to the desired reception band, other frequency bands are also able to pass unattenuated through the filter. In the ideal case, the passband of the filter should at least have the bandwidth of the receiving system and should generate there only minimal insertion loss. In the stop band, a filter should have high attenuation and, in the ideal case, a quick transition from the passband to the band-stop band, that is, a steep flank of the passband.
Different notch filters and band-stop filters with small stop-band widths are known, which essentially involve the interconnection of impedance elements. Such interconnections of impedance elements can be made, for example, from a ladder-type arrangement with SAW resonators, which are then arranged in series or parallel branches. For the most part, a complicated matching network is required, in order to realize, overall, a passband with reasonable bandwidth.
Another problem that can occur in SAW band-stop filters is the inability to withstand high power, which leads to the wear of electrode structures and thus to the premature failure of the corresponding filter components.
The task of the present invention is therefore to specify a band-stop filter that solves at least one of the aforementioned problems.